1. Field of the Invention
The present invention relates to a system, apparatus, and method for transmitting and receiving information using a multi-carrier communication method (such as OFDM (Orthogonal Frequency Division Multiplex)).
2. Description of the Related Art
The OFDM method is known as a modulation scheme to transmit an image at a high speed. The OFDM method is a multi-carrier modulation scheme which transmits multiplexed digital modulated waves having typically tens to several hundreds, sometimes to several thousands, depending on a system, of orthogonal carrier frequencies.
This modulation scheme is insensitive to selective fading, and uses a DFT (Discrete Fourier Transform) to generate multi carriers or FFT (Fast Fourier Transform) that performs a high-speed processing.
The communication using the OFDM typically inserts a repetition pattern called a guard interval in a transmission signal, thereby reducing the effect of multi-path fading.
A demodulated signal is subject to a variation in amplitude and phase rotation under the effect of multi-path fading and the effect of OFDM symbol demodulation timing. To eliminate these effects, a correction process is required. In the correction process, a known pattern symbol, called a reference symbol, is inserted into a transmission signal. In a typical method, a receiver side estimates transmission path characteristic, and multiplies a data symbol in succession to the reference symbol by an inverse version of the estimated transmission path characteristic.
FIG. 24 is a block diagram illustrating a conventional OFDM communication apparatus 2401. The communication apparatus 2401 includes a transmission terminal 2402A, encoder 2403 connected to the transmission terminal 2402A, mapper 2404 connected to the encoder 2403, inverse fast Fourier transform (hereinafter referred to as IFFT) unit 2405 connected to the mapper 2404, frequency converter (hereinafter referred to RF) 2406 connected to the IFFT 2405, fast Fourier transform (hereinafter referred to FFT) unit 2407 connected to the RF unit 2406, correction value calculator 2408 connected to the FFT unit 2407, correction processor 2409 connected to both the FFT unit 2407 and correction value calculator 2408, demapper 2410 connected to the correction processor 2409, demodulator 2411 connected to the demapper 2410, and reception terminal 2402B connected to the demodulator 2411. An antenna 2412 is connected to the RF unit 2406.
The transmission terminal 2402A receives a transmission data signal from the outside. The encoder 2403 receives the transmission data from the transmission terminal 2402A, and performs a convolution process to the transmission data at a designated encoding rate, thereby outputting an encoded data signal.
Upon receiving the encoded data signal from the encoder 2403, the mapper 2404 performs a transformation process (hereinafter referred to as a mapping process) to transform the encoded data signal into a complex symbol (a quadrature phase shift complex symbol, for example) every plurality of bits (every 2 bits, for example), and outputs the complex symbol.
The IFFT unit 2405 performs an inverse fast Fourier transform on the complex symbol obtained in the mapping process of the mapper 2404, thereby generating an OFDM modulated signal.
The RF unit 2406 receives the OFDM modulated signal supplied from the IFFT unit 2405, and frequency converts the signal into a predetermined carrier frequency assigned for signal transmission. Upon receiving frequency converted OFDM modulated signal from the RF unit 2406, the antenna 2412 radiates the corresponding radio wave in the air.
The antenna 2412 receives a radio signal transmitted from another communication apparatus and feeds the signal to the RF unit 2406. The RF unit 2406 frequency converts the received carrier frequency into an intermediate frequency signal, which is then fed to the FFT unit 2407. The FFT unit 2407 obtains an OFDM modulated signal from the received signal that is now the intermediate frequency signal, and Fourier transforms the OFDM modulated signal into a complex symbol in a frequency domain. The correction value calculator 2408 estimates a transmission path based on the output from the FFT unit 2407, thereby calculating a correction value. The correction processor 2409 receives data representing the correction value from the correction value calculator 2408. In response to the data, the correction processor 2409 performs a correction process to eliminate an amplitude variation and phase rotation from the complex symbol output from the FFT unit 2407. The demapper 2410 demaps the complex symbol into a bit, thereby reproducing the data symbol. The demodulator 2411 receives the data symbol, and performs a demodulation process on the symbol at a designated encoding rate, thereby reproducing transmitted data. The reception terminal 2402B serves the function of receiving the reproduced data and outputting received data.
The encoder 2403, mapper 2404, demapper 2410, and demodulator 2411 are all supplied with information concerning an encoding rate, modulation scheme, and carrier assignment. These units are thus operated in response to the supplied information. In a typical setting, each of the encoding rate and modulation scheme is one type and fixed per burst signal, and the carrier assignment is typically fixed in a communication system.
A transmission signal may reach a receiver side in a distorted form under the influence of the transmission path, becoming a reduced power at the receiver side. The correction value calculator 2408 and correction processor 2409 correct the received signal to be a signal power up to a desired level, and the corrected signal is then used by the demapper 2410 connected to the correction value calculator 2408 and correction processor 2409. In the conventional apparatus, the demapper 2410 and demodulator 2411 may process a carrier (received signal) in an extremely poor state with a signal to noise ratio remaining unimproved. A demodulated signal is thus adversely affected, and data is subject to error even after an error correction operation.
If reproduced data still contains an error even after an error correction operation is performed on a signal subsequent to a correction, the receiver side typically issues, to a transmitter side, a request to retransmit data in an attempt to perform correct data communication.
FIG. 25 is a data flow diagram illustrating an operation of two communication apparatuses 2401, having the construction shown in FIG. 24. A transmitter side communication apparatus (hereinafter referred to a communication apparatus A) transmits data to a receiver side communication apparatus (hereinafter referred to a communication apparatus B). Here, the communication apparatus B requests the communication apparatus A to retransmit data.
The communication apparatus A transmits data A to the communication apparatus B, and the communication apparatus B receives the data A (step S2501). The communication apparatus B performs a series of receiving steps including a fast Fourier transform, correction process, demapping process, and demodulation (step S2502). If any error is found in reproduced data, the communication apparatus B requests the communication apparatus A to retransmit the data A (step S2503). If no error is found in the reproduced data, the communication apparatus B transmits a reception end notice (ACK) of the data A to the communication apparatus A.
Upon receiving the request to retransmit, the communication apparatus A transmits the data A to the communication apparatus B again in response (step S2504). The communication apparatus B performs a series of receiving steps including fast Fourier transform, correction process, demapping process, and demodulation (step S2505). If no error is found, the communication apparatus B transmits a reception end notice (ACK) of the data A to the communication apparatus A (step S2506). If any error is found, the communication apparatus B issues a request to retransmit the data A to the communication apparatus A.
Upon receiving the reception end notice (ACK) in step S2506, the communication apparatus A transmits data B subsequent to the data A (step S2507).
The transmitter communication apparatus determines which data to transmit, in response to the reception end notice or request to retransmit coming in from the receiver communication apparatus. In this method, correct data is communicated. As the number of requests to retransmit increases, data transmission rate becomes slow.
Many errors may be found in reproduced data even after an error correction is performed on the signal subsequent to the correction process, and many requests to retransmit may occur. A method typically available in such a case is a fallback method. In the fallback method, a receiver communication apparatus requests a transmitter communication apparatus to modify a modulation scheme of a subcarrier to reduce the number of phases, or to reduce the encoding rate for an error correction to raise an error correction ability. As a result, communication is performed at a reduced transmission rate.
FIG. 26 illustrates a data flow of a conventional fallback process. The operation of the fallback process is discussed below.
The communication apparatus A sends data A to the communication apparatus B (step S2601). Upon receiving the data A, the communication apparatus B performs a series of receiving steps including fast Fourier transform, correction process, demapping process, and demodulation, and then determines, based on the error status of the reproduced data, whether to perform the fallback process (step S2602). If the communication apparatus B determines that the fallback process is required, the communication apparatus B transmits a fallback request to the communication apparatus A (step S2603). If the communication apparatus B determines that no fallback process is required, the communication apparatus B transmits a reception end notice (ACK) to the communication apparatus A.
Upon receiving the fallback request, the communication apparatus A transmits data B in succession to the data A with the modulation scheme and encoding rate modified (step S2604). Upon receiving the data B, the communication apparatus B performs a series of receiving steps including fast Fourier transform, correction process, demapping process, and demodulation, and then determines again, based on the error status of the reproduced data, whether to perform the fallback process (step S2605) If the communication apparatus B determines that no fallback process is required, the communication apparatus B transmits a reception end notice (ACK) to the communication apparatus A (step S2606). If the communication apparatus B determines that the fallback process is required, the communication apparatus B transmits a fallback request to the communication apparatus A.
Upon receiving the reception end notice (ACK), the communication apparatus transmits data C in succession to the data B (step S2607).
In response to the fallback request from the receiver communication apparatus B, the transmitter communication apparatus A modifies the modulation scheme and encoding rate of the data to be transmitted next. The generation of errors is controlled to perform correct communications in this way. However, the fallback request leads to a slow data transmission rate.
To maintain quality of communication from the standpoint of data error in the conventional OFDM communication, data itself may be retransmitted. To increase robustness to data error on a per OFDM subcarrier unit basis, the modulation scheme may be changed to the one which has the reduced number of phases, or the encoding rate of the error correction function may be reduced. These remedies in turn leads to the drawback of a reduced transmission rate.
If it is determined that an error is contained in data received by a communication apparatus, a request to retransmit the same data is repeated until the data is correctly received. The transmission rate is thus reduced. To increase robustness to the error, for example, a current “modulation scheme of 16 QAM with an encoding rate of ½” may be modified to a “modulation scheme of QPSK with an encoding rate of ¾” in response to a fallback request. Such a modification reduces the transmission rate to three-thirds the preceding modulation scheme.
Similarly, when a current “modulation scheme QPSK with an encoding rate of ¾” is modified to a “modulation scheme QPSK with an encoding rate of ½” in response to a fallback request, the transmission rate is reduced to two-thirds the preceding modulation scheme.